My 87 944S Runs Rough at Low RPM, Stalls at Idle, Help!

[notes928]

Hi All,

Having studied every Porsche forum and having suffered endless failures to find the problem i'm at my wits end with my 87 944 S. Symptoms as follows:

- Extremely rough idle or wont idle most of the time, often stalls as soon as the car is started unless accelerator is applied
- Can be coaxed to run and drive if the revs are kept up. At high revs it pulls strongly at times but sometimes its impossible to get it into 3,000 plus RPM without stalling out first
- Any load like going up a small hill will cause it to sputter and it will backfire into the intake or even afterfire into the exhaust
- Switching ignition off and restarting the car seems to reset things a bit and the car will run again for a few minutes, albeit poorly and with an extremely rough idle

Done so far:

- Compression tested OK with all cylinders between 180 to 185 psi warm
- Replaced plugs and leads
- Replaced TPS
- Replaced engine Temp sensor
- Injectors cleaned and flow tested
- New fuel pressure regulator
- Checked vacuum lines, all appear ok
- New dipstick o-rings
- Replaced throttle body O-rings. Appears to be no more vacuum leaks
- Recently new\reconditioned AFM installed by previous owner
- Recently new\reconditioned DME installed by the previous owner

Any thoughts or tips?

Thanks!

[944 Ecology]

smoke test for a vacuum leak. Sounds like a big one.

[CarGuyNeil]

Is the fuel pressure within spec? Weak pump? I would definitely do what the poster above me said and check for vaccuum leaks.

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[notes928]

Originally Posted by 944 Ecology

smoke test for a vacuum leak. Sounds like a big one.

Thanks, ill try it. Where i am i don't have any competent mechanics around. I will have to try and rig up a DIY smoke machine and try a smoke test.

I believe i don't have any more leaks as i identified a few leaks previously and managed to drop my idle from a surging 1,400 RPM to around 900 RPM albeit with car cutting out at idle now which it did not do before.

[notes928]

Originally Posted by CarGuyNeil

Is the fuel pressure within spec? Weak pump? I would definitely do what the poster above me said and check for vaccuum leaks.

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Not sure about fuel pressure. Will have to pick up a fuel pressure gauge and check the fuel pressure as well. The car running better at high RPM might indicate that its not a fuel pressure issue but ill check it out this weekend and report back.

[CarGuyNeil]

I remember ones of Porsche 944 fix-it videos they had a leak at the throttle body, worn seal at the shaft.


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[notes928]

Originally Posted by CarGuyNeil

I remember ones of Porsche 944 fix-it videos they had a leak at the throttle body, worn seal at the shaft.


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Yeah i already replaced the throttle body shaft o-rings. They were original and had completely disintegrated.

[S944calalta]

I saw your post about an idle issue. To confirm, this is a 16 valve 944S, correct? I reviewed your list of items you have checked. Did you test your idle control valve (ICV)? It is often overlooked on the 16 valve cars. If it is malfunctioning it will cause the issue you have described. Its a Bosch part and available at Pelican part #280-140-531. The ICV is different on the S than the regular NA 944. There are tips for testing the ICV on youtube if that's any help.

[Jfrahm]

The tach should slow a low RPM bounce when cranking.

I'm not a fan of the AFM spring adjustment, I would get another AFM unless you can set this one back the way it was and maybe retrack the contacts.

I believe the car should idle OK with the AFM harness disconnected but die if you hit the throttle. It might be interesting to see if it idles better with the AFM disconnected.

[notes928]

Originally Posted by S944calalta

I saw your post about an idle issue. To confirm, this is a 16 valve 944S, correct? I reviewed your list of items you have checked. Did you test your idle control valve (ICV)? It is often overlooked on the 16 valve cars. If it is malfunctioning it will cause the issue you have described. Its a Bosch part and available at Pelican part #280-140-531. The ICV is different on the S than the regular NA 944. There are tips for testing the ICV on youtube if that's any help.

The car is a 16V 944 S.

ICV is new and tests fine.

[notes928]

Originally Posted by Jfrahm

The tach should slow a low RPM bounce when cranking.

I'm not a fan of the AFM spring adjustment, I would get another AFM unless you can set this one back the way it was and maybe retrack the contacts.

I believe the car should idle OK with the AFM harness disconnected but die if you hit the throttle. It might be interesting to see if it idles better with the AFM disconnected.

I reset the AFM spring back to factory settings since it had no effect and sealed up the unit again.

I tried starting the car again and the tach shows 7,000 rpm during cranking. This would indicate an issue with the speed sensor but i replaced this with no effect on the issue.

Any ideas on why the tach would do this? It might provide some clarity on the problem.

[blue911rsr]

Had a car from another shop with many of your issues ended up having a few issues cam to cam timing was of which means Intake cam reset that made it better but in the end it needed a new from Porsche air flow sensor the rebuild unit it had checked out ok electrical
But had mechanical binding problems spent a lot of time on the car and new sensor was pricey but runs good now
Hope that helps btw old sensor felt ok by feel

[notes928]

Originally Posted by blue911rsr

Had a car from another shop with many of your issues ended up having a few issues cam to cam timing was of which means Intake cam reset that made it better but in the end it needed a new from Porsche air flow sensor the rebuild unit it had checked out ok electrical
But had mechanical binding problems spent a lot of time on the car and new sensor was pricey but runs good now
Hope that helps btw old sensor felt ok by feel

Thats good to know. Might have to look into cam timing if all else fails.

Did the car with the bad AFM have probems idling as well? I was told that the air flow meter should not affect anything at idle speeds.

[notes928]

UPDATE: Fixed! Was a Vacuum leak which has now been corrected and the car runs perfectly. Thanks for all the helpful suggestions.